All-reflective optical systems are desirable for many wideband optical applications because they reflect all wavelengths of incident light equally, unlike refractive systems wherein the refraction is wavelength dependent. Reflective optical systems may also be made quite compact. However, reflective optical systems are typically more limited in their major-axis fields of view than are refractive systems, due to the poor image quality, image distortions, and potential obscuring of the ray paths when the wide field of view is attempted. Only one large dimension of field of view is normally required in many optical systems, in the “major axis”. A simultaneously large minor-axis field of view is either not necessary because the optical system is scanned along a direction, as in satellite-based earth-sensing applications, or because the minor-axis field of view is supplied by angularly scanning the optical system along the minor axis using a mechanical scanning device.
For example, high-image-quality reflective optical systems are available for major-axis fields of view of up to about 12-14 degrees. These optical systems typically use three or four mirrors, such as three powered mirrors and an aspheric corrector mirror. However, analysis shows that these mirror systems cannot provide good-quality optical images for fields of view greater than about 15 degrees and about 20 degrees, and certainly not for fields of view greater than about 25 degrees. In the range of these higher fields of view, the image quality is too degraded by substantial image aberrations and distortions to be acceptable.
There is a need for an all-reflective optical system having a wider field of view than available with current optical systems. The present invention fulfills this need, and further provides related advantages.